Grinding wheel

ABSTRACT

A grinding wheel is mounted at a distal end of a spindle and grinds a wafer held on a holding table. The grinding wheel includes: an annular base having a mounting surface to be mounted on the distal end of the spindle; and a plurality of segment grindstones that are fixedly attached annularly to a surface opposite to the mounting surface of the annular base and that are equidistantly spaced apart from each other. The annular base has a plurality of slits formed therein. Each of the slits represents a gap that is formed between two adjacent segment grindstones and that is extended toward a side of the annular base such that the slit has a width of the gap.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a grinding wheel that grinds a plate-shaped workpiece such as a wafer.

Description of the Related Art

In performing a grinding operation for grinding a wafer, a grinding apparatus (see, for example, Japanese Patent Laid-Open No. 2000-288881) that includes a holding table and a grinding unit is used. The holding table holds the wafer. The grinding unit includes a spindle that rotates a wheel mount in which a grinding wheel is mounted. The grinding wheel includes a plurality of segment grindstones disposed annularly therein. Then grinding is performed as follows. While grinding water is supplied to the segment grindstones, a rotating grinding wheel is lowered to thereby bring the segment grindstones into contact with an upper surface of the wafer held in the holding table.

SUMMARY OF THE INVENTION

A predetermined spacing (gap) is provided between two adjacent segment grindstones, so that the grinding water supplied to the grinding wheel can be discharged from an inner peripheral side to an outer peripheral side of the segment grindstones by a centrifugal force during the grinding operation. The gap functions as a discharge port through which the grinding water is discharged to an outside of the segment grindstones during the grinding operation.

As the segment grindstone is worn down toward a root thereof by grinding, however, the segment grindstone, now lower in height, diminishes the gap (discharge port), resulting a reduced draining effect. As a result, the grinding water including swarf tends to stagnate in an area on the inner peripheral side of the segment grindstone, and the swarf may accumulate and be deposited on the inside of the segment grindstone or may be caught by the rotating segment grindstone to be stuck on a grinding surface at a lower end of the segment grindstone. When the swarf deposited on the segment grindstone falls on the wafer or grinding is performed by a segment grindstone having the swarf sticking to the grinding surface thereof, deep scratches or scratches extending irregular directions are formed in the wafer. This unfortunately affects a device.

An object of the present invention is to provide a grinding wheel that reliably discharges grinding water outside the segment grindstone even when the segment grindstone wears and thereby does not allow swarf to accumulate inside the segment grindstone.

In accordance with an aspect of the present invention, there is provided a grinding wheel, disposed at a distal end of a spindle, for grinding a wafer held on a holding table. The grinding wheel includes: an annular base having a mounting surface to be mounted on the distal end of the spindle; and a plurality of segment grindstones that are fixedly attached annularly to a surface opposite to the mounting surface of the annular base and that are equidistantly spaced apart from each other. The annular base has a plurality of slits formed therein. Each of the slits represents a gap that is formed between two adjacent segment grindstones and that is extended toward a side of the annular base such that the slit has a width of the gap.

The grinding wheel in the aspect of the present invention can prevent scratches affecting a device from being formed on a wafer because, even when the annularly disposed segment grindstones have worn down to thereby diminish the gaps on the side of the segment grindstones to serve as grinding water discharge ports, the slits in the base function as the discharge ports and thus swarf does not stagnate in areas inside the segment grindstones.

The above and other objects, features, and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claim with reference to the attached drawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view depicting an example of a grinding wheel according to an embodiment of the present invention;

FIG. 2 is a perspective view depicting an example of a known grinding wheel;

FIG. 3 is a partly cross-sectional side elevation view depicting a state in which a wafer held in a holding table is ground by a rotating grinding wheel while grinding water is being supplied; and

FIG. 4 is a plan view schematically depicting an area of the wafer to be machined by segment grindstones during a grinding operation, as viewed from above.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A grinding wheel 1 according to an embodiment of the present invention depicted in FIGS. 1 and 3 includes a ring-shaped base 10 formed of, for example, stainless steel or aluminum. The base 10 has a flat mounting surface 100. The mounting surface 100 is mounted on a side of a distal end of a spindle 20 depicted in FIG. 3 via a wheel mount 21 depicted in FIG. 3. A surface of the base 10 opposite to the mounting surface 100 assumes a flat grindstone fixing surface 101. A segment grindstone is fixed to the grindstone fixing surface 101. The mounting surface 100 and the grindstone fixing surface 101 extend in parallel with each other. As depicted in FIG. 1, the base 10 has a circular opening 102 formed at a center thereof. The opening 102 passes through the base 10, extending from the mounting surface 100 to the grindstone fixing surface 101.

A plurality of segment grindstones 11 are fixedly attached annularly to the grindstone fixing surface 101 of the base 10 by an appropriate adhesive agent. The segment grindstones 11 are spaced equidistantly in a circumferential direction with a gap 104 of a predetermined width interposed between each pair of adjacent segment grindstones 11. The segment grindstones 11 are each formed of, for example, metal, ceramic, resin, or other binding material (bonding material) mixed with abrasive grains such as diamond and cubic boron nitride (CBN), and are each formed substantially into a rectangular parallelepiped. It is noted that the types of binding materials and abrasive grains are not restrictive and may be selected or changed according to use, for example.

The base 10 has an inner surface that assumes, for example, an inclined surface inclined at a predetermined angle. The inner surface has a plurality of grinding water supply ports 103 formed to be spaced equidistantly in the circumferential direction. Grinding water such as pure water jets out from the grinding water supply ports 103. The grinding water supplied from the grinding water supply ports 103 flows over the inclined surface to thereby cool the base 10. The grinding water further cools the segment grindstones 11 and a portion of a wafer W (see FIG. 3) being ground, thereby removing generated swarf from a back side Wb of the wafer W. The base 10 is not necessarily required to have the grinding water supply ports 103 formed therein.

Unlike a base 10 of a grinding wheel 1A of the known art depicted in FIG. 2, the base 10 of the grinding wheel 1 in the embodiment of the present invention has a plurality of slits 106 formed therein. The slits 106 each represent the gap 104 formed between two adjacent segment grindstones 11, the gap 104 extended toward the side of the base 10 so as to have a width of the gap 104. The slit 106 depicted in the example of FIG. 1 has a length extending to a position that is substantially a middle of a thickness of the base 10. This configuration is, however, illustrative only and not limiting.

Reference is made to FIG. 3. The grinding wheel 1 is mounted at the distal end of the spindle 20 via the circular wheel mount 21 for use. The spindle 20 has an axial direction extending in a Z-axis direction and is rotatable about an axis in the Z-axis direction by a motor not depicted.

As depicted in FIG. 3, the spindle 20 has a flow path 20 a formed thereinside. The flow path 20 a assumes a path through which the grinding water passes. The flow path 20 a passes through the spindle 20 in the axial direction (Z-axis direction). The flow path 20 a communicates with flow paths 21 b formed in the wheel mount 21. A grinding water supply source 25 is disposed upstream of the flow path 20 a to communicate with the flow path 20 a. The grinding water supply source 25 supplies the grinding water. The flow paths 21 b are disposed to be spaced apart from each other at predetermined intervals in the circumferential direction in the wheel mount 21 in directions orthogonal to the axial direction of the spindle 20 inside the wheel mount 21. The flow paths 21 b communicate with respective grinding water supply ports 103 in the base 10 of the grinding wheel 1.

A holding table 30 holds the wafer W. The holding table 30 has a circular profile, for example. The holding table 30 includes a suction portion 300 and a frame body 301. The suction portion 300 is formed of, for example, a porous material and draws the wafer W by suction. The frame body 301 supports the suction portion 300. The suction portion 300 communicates with a suction source, not depicted, and holds the wafer W by suction on a holding surface 300 a that is an exposed surface of the suction portion 300. The holding surface 300 a is formed into a conical surface that is inclined extremely mildly to have a center of rotation of the holding table 30 as an apex. Rotary means 31 is connected to a bottom surface side of the holding table 30. The holding table 30 is rotatable axially about the Z-axis direction by the rotary means 31. Additionally, the holding table 30 is capable of reciprocating motion in a Y-axis direction by moving means not depicted.

A grinding water jet nozzle 38 is disposed so as to face the inner surface of the base 10 and inner surfaces of the segment grindstones 11. The grinding water jet nozzle 38 squirts grinding water supplied from a grinding water supply source 39 toward contact portions between the segment grindstones 11 and the wafer W from the side of the inner surface of the rotating grinding wheel 1, to thereby cool the contact portions and remove the swarf generated by grinding.

The following describes grinding of the wafer W depicted in FIG. 3 using the grinding wheel 1. The wafer W may, for example, be a semiconductor wafer using silicon as a base material and having a circular plate-shaped profile. A plurality of devices are formed on a front side Wa. The front side Wa faces downward in FIG. 3 and is protected by protective tape not depicted. The back side Wb of the wafer W assumes a ground surface on which the grinding operation is performed. The wafer W may still be, for example, a resin substrate or a ceramic substrate.

The holding table 30, which holds the wafer W with the back side Wb exposed to the upper side, is moved in a +Y direction to a position below the grinding wheel 1 by a moving mechanism not depicted. Alignment is then performed between the grinding wheel 1 and the wafer W held in the holding table 30. The alignment is performed, for example, such that the center of rotation of the grinding wheel 1 is offset by a predetermined distance in the +Y direction with respect to a center of rotation of the wafer W and a rotational trajectory of the segment grindstones 11 passes through the center of rotation of the wafer W. In addition, inclination of the holding table 30 is adjusted such that the holding surface 300 a, which is a mildly inclined conical surface, is in parallel with grinding surfaces (lower surfaces) of the segment grindstones 11. The foregoing adjustment causes the back side Wb of the wafer W that is held by suction onto and along the holding surface 30 a as a conical surface to extend in parallel with the grinding surfaces of the segment grindstones 11.

After the alignment of the grinding wheel 1 with the wafer W has been performed, the grinding wheel 1 rotates counterclockwise as viewed from the side of a +Z direction as depicted in FIG. 3, as the spindle 20 is rotationally driven by a motor, not depicted. Furthermore, the grinding wheel 1 lowers in a −Z direction to thereby bring the segment grindstones 11 into abutment on the back side Wb of the wafer W, so that grinding is performed. During the grinding, the wafer W rotates as the holding table 30 rotates counterclockwise as viewed from the side of the +Z direction, so that the segment grindstones 11 grind an entire surface of the back side Wb of the wafer W.

It is noted that, because the wafer W is held by suction onto and along the holding surface 300 a as the conical surface, the segment grindstones 11 abut on and grind the wafer W as depicted in FIG. 4 over a range of the rotational trajectory of the segment grindstones 11 indicated by an arrow R in FIG. 4.

During the grinding operation, the grinding water supply source 25 depicted in FIG. 3 supplies grinding water for mainly cooling the grinding wheel 1 to the flow path 20 a in the spindle 20. The grinding water supplied to the flow path 20 a passes through the flow paths 21 b in the wheel mount 21 and jets from the grinding water supply ports 103, reaching the segment grindstones 11. In addition, grinding water is squirted from the grinding water jet nozzle 38 toward the contact portions between the segment grindstones 11 and the wafer W from the side of the inner surface of the rotating grinding wheel 1.

The grinding water jetted from the grinding water supply ports 103 and the grinding water squirted from the grinding water jet nozzle 38 are discharged outside the segment grindstones 11 through the gap 104 in each of the segment grindstones 11 depicted in FIG. 1 by a centrifugal force generated from rotation of the holding table 30, together with the swarf and abrasive grains, etc., dropped off from the segment grindstones 11, before flowing over the back side Wb of the wafer W down from the holding surface 300 a of the holding table 30.

The grinding operation performed by the grinding wheel 1 for a single or a plurality of wafers W as described above causes the segment grindstones 11 to wear down toward the roots thereof to be shorter in height. When, for example, the wafer W is ground with the grinding wheel 1A in the known art depicted in FIG. 2, the segment grindstone 11, which is now shorter in height, diminishes the gap 104, resulting in a reduced grinding water draining effect. As a result, the grinding water stagnates in the range indicated by the arrow R in FIG. 4, so that swarf may accumulate and be deposited on the inside of the segment grindstone 11 or may stick to the grinding surface (lower end surface) of the segment grindstone 11. When the swarf deposited on the segment grindstone 11 falls on the wafer W or grinding is continuously performed by the segment grindstone 11 having the swarf sticking to the grinding surface thereof, deep scratches or scratches extending irregular directions are formed in the wafer W.

In the grinding wheel 1 in the embodiment of the present invention, however, even when the gap 104 diminishes due to the segment grindstone 11 worn down through the performance of the grinding operation, the slit 106 in the base 10 functions as a discharge port to discharge the grinding water outside the segment grindstone 11. Thus, the grinding water including the swarf does not stagnate in the range indicated by the arrow R in FIG. 4 inside the segment grindstones 11, so that scratches affecting the device can be prevented from being formed on the wafer W.

The grinding wheel 1 in the embodiment of the present invention is useful for an inner cutter grinding operation in which the grinding wheel 1 and the holding table 30 rotate in an identical rotating direction as in the embodiment described above and the rotating segment grindstones 11 access an outer periphery of the wafer W to perform grinding toward the center of the wafer W. The grinding wheel 1 in the embodiment of the present invention is also useful for a grinding method called TAIKO grinding. The TAIKO grinding is a type of grinding method for improving handling performance while grinding the wafer W to an ultrathin thickness. The grinding method uses a grinding wheel having a diameter smaller than a diameter of the wafer W. The grinding method forms a circular recess by grinding an area on the back side Wb of the wafer W corresponding to an area, in which devices are formed, on the front side Wa of the wafer W and then forms an enforcement annular protrusion in an outer peripheral area on the back side Wb of the wafer W.

During the TAIKO grinding operation, the segment grindstones contact the wafer W via an area greater in ratio than in the ordinary grinding operation described previously. Thus, compared with the ordinary grinding operation, the TAIKO grinding tends to reduce draining efficiency of the grinding water outside the segment grindstones. Moreover, the gap between the segment grindstones diminishes as the segment grindstones wear down, which results in further reduced draining efficiency of the grinding water. The slit is formed in the base of the grinding wheel having a diameter smaller than the diameter of the wafer W used in the TAIKO grinding by extending the gap between the segment grindstones toward the base side such that the slit has the width of the gap. This configuration enables the grinding water to be drained even more reliably during the TAIKO grinding.

The present invention is not limited to the details of the above described preferred embodiment. The scope of the invention is defined by the appended claim and all changes and modifications as fall within the equivalence of the scope of the claim are therefore to be embraced by the invention. 

What is claimed is:
 1. A grinding wheel, disposed at a distal end of a spindle, for grinding a wafer held on a holding table, the grinding wheel comprising: an annular base having a mounting surface to be mounted on the distal end of the spindle; and a plurality of segment grindstones that are fixedly attached annularly to a surface opposite to the mounting surface of the annular base and that are equidistantly spaced apart from each other, wherein the annular base has a plurality of slits, each of the slits representing a gap that is formed between two adjacent segment grindstones and that is extended toward a side of the annular base such that the slit has a width of the gap. 